1. Field of the Invention
The present invention concerns the thermal protection of structural surfaces in the presence of an erosive flow. The invention is more particularly concerned with a thermal protection device and a method of fabricating the latter, that is intended to protect structural surfaces exposed to an erosive, and possibly corrosive, flow of gas at high speed and high temperature and to high levels of vibration, in particular when this is inherent to their operation. It is to be understood that the high-speed flow is a relative flow in the sense that it can be the movement of a vehicle in a gaseous atmosphere or a high-speed gaseous flow relative to a fixed structure, for example in a propulsion nozzle.
2. Description of the Prior Art
A routine way to protect a surface from such flow is to use ablative (or ablatable) thermal protection, i.e. a coating that protects the surface and is consumed. This type of protection naturally applies only to short term exposure (in practice a few hundred seconds maximum).
The thermal protection layers of structures to be protected from an erosive flow are usually made up of composite materials with organic, organo-metallic or mineral binders capable of including powder, fiber, organic or mineral woven reinforcements. Due to the action of the hot gases, the ablatable material is subjected to the phenomenon of pyrolysis. This pyrolysis is accompanied by degradation of the carbon-containing or organo-silicic chain of the binder that renders the slag fragile and liable to break. To overcome this the thermal protection layer is routinely reinforced using various techniques.
The abrasion of an ablatable material of this type exposed to erosion by hot gases and to vibration is known to be reduced if the material is reinforced. The reinforcement embedded in the insulative material can be a metal or non-metal, woven or fibrous. There is less ablation if the fibrous or woven reinforcement is anchored and oriented in the matrix perpendicularly to the direction of the gases and there is less heat transfer if the reinforcement is oriented in the direction of the gases. The fibrous or woven reinforcements appear to offer less thermal insulation than the insulative matrix.
There are two main types of methods of making thermal protection materials:
1) a first type of method uses compression at very high pressure (typically several hundred bars) of a mass of reinforcing fibers pre-impregnated with resin; this is a kind of molding with injection of the pre-impregnated fiber mass between two half-shells; and
2) a second type of method involves winding on a fiber impregnated with a resin, impregnation preferably being effected during the winding operation.
The winding method has the advantage of not requiring such high pressures as are required by the compression method; the winding method can include a pressurized step, but the pressure in question is typically in the order of approximately ten bars at most.
Examples of thermal protection devices are given by the documents EP-0.174.886, EP-0.398.787, FR-2.652.036, EP-0.471.605 and EP-0.501.861, covering prior inventions made by the Applicant.
Document EP-0.174.886 discusses thermal protection including an insulative polymerized resin layer fixed to the surface of the wall to be protected; this layer includes an armature having a fringed mesh with a mesh part exposed to the erosive flow and fringes directed, with a predetermined inclination, towards the surface to be protected. Generally speaking, this disclosure concerns the protection of hollow structures with a monotonously changing section.
Prior art document EP-0.398.787 proposes an improved solution to the above disclosure in the sense that it teaches thermal protection including, as in the previous solution, a refractory armature formed of a fringed mattress with a mesh part exposed to the erosive flow and fringes adapted to be directed towards the structure wall to be protected. The armature is embedded in a thermally insulative matrix. This thermal protection further includes a wide-mesh woven refractory material disposed parallel to the mesh part of the armature with the fringes passing through the wide mesh. The document describes a protection layer, advantageously a refractory protection layer, facing the ends of the fringes and which in practice includes a wound filament or tape binding the woven refractory material. This layer is advantageously eliminated after curing, during final machining of the thermal protection, so that in practice none of it remains during use of the thermal protection.
Prior art document FR-2.652.036 proposes a thermal protection coating having a different structure in that it includes a main layer formed of a succession of refractory fibrous reinforcement slices substantially parallel to each other but inclined to the surface to be protected and between which insulative slices are interleaved, this main layer being lined with at least one sub-layer extending along the surface to be protected and essentially consisting of an insulative material compatible with that of the insulative slices. The refractory fibrous reinforcement is in practice a tape. The sub-layer is formed of the same insulative material as the insulative slices, for example. This sub-layer serves mainly to anchor the fibrous refractory reinforcement since, during the fabrication of the thermal protection coating, the latter is engaged in grooves formed in this sub-layer. The latter can also have other functions, such as protection against X-rays or super-insulation using cellular material.
A third type of thermal protection is proposed in document EP-0.471.605 wherein thermal protection is obtained by winding a plurality of superposed layers of plush refractory filaments around a mandrel temporarily provided with radial barbs and reinforcing pins, also made of refractory material, which form an integral part of the finished thermal protection.
Finally, document EP-0.501.861 proposes three-dimensional thermal protection as in the previous document formed of a stack of impregnated woven materials traversed by refractory material fibers.
These various solutions constitute undoubted progress. Nevertheless, there is a need for non-ablative thermal protection resistant to an erosive flow of gas at high speed and to severe thermal aggression, either for longer time periods or for the same time periods, for parts where the geometry must remain unchanged.
An object of the invention is to meet this need.